Direction finding becomes a more difficult task giving the general use of more exotic communications systems utilizing spread spectrum, monopulse, packet, or other jam-resistant techniques In the past, some of the techniques for direction finding have included the use of multiplexed antennae coupled to a single receiver to obtain a phase comparison to determine the direction of arrival of the signal. Other techniques for direction finding have included the use of directional antennae with a phase comparison performed at the output of two receivers which are calibrated together. This technique has various disadvantages including erroneous direction of arrival measurements due to modulation rates or system errors because of lack of calibration. Common disadvantages are that they are slow, and have difficulty in determining the angle of arrival of spread spectrum signals and monopulse or packetized signals.
Recently, the use of a Chirp-Z Transform Discrete Fourier Transform has allowed direction finding of multiple frequencies simultaneously over wide bandwidths. The use of a Chirp-Z Transform with Direction Finding Apparatus has been disclosed in U.S. Pat. No. 4,443,801. As set forth in that patent, signals of interest are received, heterodyned, and then mixed with the sweeping chirp local oscillator. Mixed signals are coupled to two Chirp-Z transformer channels The outputs of the two Chirp-Z transformer channels are fed to a phase comparator circuit that resolves the relative phase shift between the signals from each of the channels The frequency of the input signals is derived from the time ordering of the energy spikes, these spikes being the physical representation of the transformed coefficients, present at the output of the Chirp-Z channels prior to phase detection. The angle of arrival of a signal of given frequency is determined by sampling of the phase comparator outputs followed by a simple calculation performed by a microprocessor. The input receivers in this patent process the RF input signals and generate IF output signals. The signals are then mixed with a known frequency to obtain a difference frequency in each channel separated by a known frequency. The signals in each channel are then amplified and applied to a surface acoustic wave dispersive delay line. The resultant output signal is a pair of very narrow pulses, the width of which is inversely proportional to the bandwidth of the dispersive delay line. Thus the output signal from the surface acoustic wave dispersive delay line is a pair of RF pulses of different nominal center frequency separated in the time domain as distinguished from the input signal into the delay line which comprised two spread spectrum systems, overlapping in frequency but coincident in time. This type of signal transformation is called a Chirp Transform which is well known in the art.
There are several problems with this type of prior art direction finding apparatus. In the first place, a local oscillator is used to down convert each input RF signal to an IF signal which is injected into the dispersive delay line. It is well known that SAW devices change operational characteristics as a function of temperature, as well as age, and they must be recalibrated periodically. The dispersive delay line devices of the prior art utilize both RF and IF circuits, both of which are temperature sensitive. Also, such circuits can cause amplitude modulation to be converted t phase modulation. The SAW devices compare the phase of the received signals. The angle or phase between the two channels is directly related to the difference in time of arrival of the signal at the two antennae. That time is very small, in the order of 10 ns for a 10' separation of the antenna. The length of the SAW delay line may be 20 .mu.s or 20,000 ns. Thus, very precise control of the signal phase in each channel is required in order not to lose the 10 ns phase difference out of the 20,000 ns delay in the SAW delay line. This is the reason the system is so temperature sensitive because a small change in temperature can distort the output. In addition, the dispersive delay lines have a fixed delay and thus the frequency resolution of the device cannot be adjusted.
The present invention overcomes the disadvantages of the prior art by including receivers for down converting the incoming RF signals to baseband thereby avoiding any further temperature considerations in the remainder of the circuit. It is only in the RF receiver portion that temperature sensitivity will have any effect and that is a small effect. Secondly, the delay line of the present device is formed with a charge coupled device (CCD) which receives the baseband orthogonal I and Q signals. The charge coupled device enables the use of a clock frequency common to the two channels to control the amount of delay. Thus, the charge coupled device becomes a variable delay line whose delay or length depends upon the clock frequency setting the sampling rate. The faster the sampling rate, the higher the resolution. Thus, the Chirp-Z transform implemented with a charge coupled device delay line technology using variable sampling rates and baseband signals for processing by the delay line overcomes the problems of the prior art.
The present invention provides a direction finding apparatus in which the receivers coupled to the separated antennae reduce the incoming RF signals to baseband signals for processing by the Chirp-Z Transform Discrete Fourier Transform circuit.
The present invention also provides a direction finding apparatus which utilizes a charge coupled device as a variable length delay line associated with the Chirp-Z transform.
The present invention still further provides a direction finding apparatus in which a control circuit provides a variable sampling rate to the charge coupled device delay line to allow the adjustment of the frequency resolution of the charge coupled device Chirp-Z transform apparatus.